During the combustion of fossil fuels, sulphur in the fuel is released in form of sulphur oxides (SOx). Other pollutants are primary particulate matter, such as soot, oil and heavy metal particles, and nitrogen oxides (NOx). It is well known that air pollution seriously impact people's health and the environment. It is also well known that sulphur dioxide and nitrogen oxides are the major precursors of acid rain.
Present regulations regarding emission control for international shipping include caps on the sulphur content of fuel oil as a measure to control SOx emissions. Special fuel-quality provisions exist for SOx in emission control areas and there are substantial reductions in the allowed fuel sulphur limits to be expected in the near future. The MARPOL Annex VI legislation, which went into effect in May 2005 following specifications from several European Union directives, has curbed the impact of marine diesel on the environment. By 2015, the legislations will be even stricter regarding by way of example fuel sulphur limits and NOx cap.
There are different emission reduction possibilities, either taken alone or in combination. One possibility is to use new fuels such as distillate fuels or low sulphur fuels. Another possibility is to further develop methods to control the emission of SOx such as wet scrubber technologies, typically used on-board ships, using alkaline agents such as NaOH-solutions, or dry scrubber technologies using granulates of hydrated lime (Ca(OH)2).
It is well known in the marine industry today for the purpose of reducing SOx in the exhaust from a ship engine to apply Exhaust Gas Cleaning (EGC).
One well known wet scrubber of the EGC type is a so called closed loop scrubber which uses circulating freshwater in combination with an alkaline agent like sodium hydroxide (NaOH) or sodium carbonate (Na2CO3) to wash out sulphur oxides and soot particles from the exhaust gas. To control the quality of the circulating freshwater a small amount of it may occasionally or continuously be replaced by clean fresh water and either be stored on the ship or be discharged over board after cleaning.
Although scrubbers of the above mentioned type are well known in the art, several unsolved or problematic issues remain. Water consumption of a closed loop scrubber system due to evaporation is generally so high that large amounts of fresh water have to be added continuously to the system to keep it in balance. Further, water cleaning in a closed loop scrubber system is critical. If the water is too dirty it is not allowed to be discharged, and it can be difficult to avoid a build-up of soot inside the scrubber system which may eventually block valves and nozzles and cause malfunction of scrubber system components. Also, the consumption of pH neutralizing chemicals like NaOH or Na2CO3 is so large that it becomes expensive to operate the scrubber system.
EP 1 857 169 A1 discloses a fresh water scrubber system comprising a two section scrubber, wherein the first section is intended for sulphur removal whereas the second section is intended for condensation.